Effect of Structure and Functional on the SFG Spectrum at the Alumina-Water Interface

The structure and dynamics of water at mineral interfaces is often hotly contested, as it can be impacted by
subtle differences in the structure and orientation of surface OH groups. The surface-specific vibrational spectroscopy method Sum Frequency Generation (SFG) is often used to study the structure of interfacial water, but SFG cannot distinguish between water and surface vibrational modes, and the structure is indirectly inferred from vibrational features. Thus, we use DFT-MD to study the structure and SFG spectrum of the (001) and (110) alumina-water interfaces, showing the former is more ordered than the latter, in contrast to current interpretations of XRR and SFG data. We the investigate the effect of the density functional on our results, comparing PBE, RPBE, PBE+TS, and SCAN for the (001) interface. While all functionals reproduce general features of the spectrum, the broadening and variability between simulations can be strongly impacted by choice of functional, with SCAN providing the best description of the interface.